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LAKE TAHOE — Lake Tahoe’s average clarity increased substantially in 2011, but the annual measurement indicating the lake’s general health was a tale of two seasons.

The average Secchi depth for the lake was 68.9 feet in 2011, an improvement of 4.5 feet over 2010, according to the UC Davis Tahoe Environmental Research Center’s 2012 State of the Lake Report, released Thursday.

Secchi depths are gathered by lowering a dinner-plate sized white disc into the lake and recording when the plate is no longer visible to observers. Lake Tahoe’s Secchi depth was 102.4 feet in 1968.

The gains in average annual clarity came from December through March, when clarity increased to 84.9 feet, an improvement of 12 feet compared to 2010, according to the report.

But measurements put the lake’s average summer clarity at 51.4 feet — the second lowest transparency ever recorded for the lake.

“Unlike the winter clarity pattern, where there is a long-term trend of declining and then improving clarity, the summer trend is dominated by a consistent long-term decline but with a noticeable 10-15 year cyclic pattern,” according to the report.

Researchers are examining the reasons behind the summer clarity decrease. There is some evidence that increasing numbers of Cyclotella, a tiny algae that scatters light and decreases clarity, is playing a role in summer clarity declines, according to the report.

“The tiny cells have grown exponentially in the past five years, scattering light and reducing clarity,” according to a statement from UC Davis. “Research shows that times of the highest concentrations of Cyclotella coincide with the lowest summer clarity levels.”

This year’s average clarity measurement was a surprise to researchers, who expected 2011’s massive snowfall to wash above average amounts of sediment and nutrients into the lake.

“This year we were expecting it to go down and it didn’t,” said Geoffrey Schladow, director of the Tahoe Environmental Research Center. He also warned against putting too much emphasis on a single year’s clarity measurement, saying long-term trends are more important indicators when looking at how to manage the lake.

Researchers cannot point to a single cause for this year’s unexpected finding, but fundamental shifts in the environment, like climate change and the lake’s hydrological cycle, may be playing a role, Schladow said.

One example noted in the report, is that during the past 43 years, the length of time a summer-like water layering in the lake persists has increased by almost 20 days, a likely outcome of climate change. Researchers fear that if this trend continues, oxygen replenishment to the bottom of the lake will become less frequent.

Without periodic mixing, swaths of the lake could become inaccessible to fish. Nutrients that have been locked up for long periods of time at the bottom of the lake could also be released in the absence of oxygen, Schladow said, a situation referred to as “internal loading.”

The report notes the importance of managing both fine sediment and nutrients that fuel algae growth in improving Lake Tahoe’s clarity.

Although a reduction in the amount of fine sediment particles entering the lake from stormwater management efforts may be part of the reason behind the winter clarity gains, a “comprehensive, regional urban stormwater monitoring plan” is needed to determine whether stormwater projects have actually reduced sediment loading, according to the report.

In addition to clarity, the report contains data on algae growth, invasive species and the effects of climate change on precipitation, lake temperature and density stratification.